Unlike in the human eye, the thousands of tiny lenses, which make the compound eye’s characteristic net-like surface, do not move, or cannot accommodate. But the University of Sheffield researchers found that photoreceptor cells underneath the lenses, instead, move rapidly and automatically in and out of focus, as they sample an image of the world around them. This microscopic light-sensor “twitching” is so fast that we cannot see it with our naked eye. To record these movements inside intact insect eyes during light stimulation, the researcher had to build a bespoke microscope with a high-speed camera system.
Remarkably, they also found that the way insect compound eye samples an image (or takes a snapshot) is tuned to its natural visual behaviours. By combining their normal head/eye movements - as they view the world in saccadic bursts - with the resulting light-induced microscopic photoreceptor cell twitching, the insects, such as flies, can resolve the world in much finer detail than was predicted by their compound eye structure, giving them hyperacute vision.